Apparatus for generating an optical interference pattern

ABSTRACT

Apparatus for producing an interference pattern from an input optical beam, including a first optical element for separating the input optical beam into a plurality of divergent optical sub-beams and a second optical element including a first surface and a second surface. The first surface is optically coupled to the first optical element to receive at least two of the plurality of sub-beams. In addition, the second optical element is capable of redirecting via total internal reflection at least one of the sub-beams received at the first surface such that at least two sub-beams emerge from the second surface along respective paths intersecting one another outside the second optical element at a distance from the second surface. Advantageously, the use of total internal reflection in the second optical element for sub-beam redirection removes any requirement for a mirror to perform this function, results in only minimal power losses as the sub-beams reflect internally and the location at which they intersect is easily adjustable.

This application is a Continuation of U.S. application Ser. No.10/296,079, filed Nov. 19, 2002, which is the National Stage ofPCT/CA02/01184, filed Jul. 26, 2002 and which application(s) areincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to the generation of optical interferencepatterns, which can be of particular use in producing changes in theindex of refraction of a glass medium such as the core of an opticalfiber.

BACKGROUND OF THE INVENTION

Certain types of glass have optical properties that can be altered whenthey are exposed to radiation. In particular, some index of refractionvariations can be permanently inscribed in these types of glass withultraviolet radiation. In Bragg grating writing by flood exposure, aninterference pattern is permanently written in the form of index ofrefraction variations in an optical fiber. Typically, a laser beam inthe ultraviolet (UV) region of the optical spectrum is split into twosub-beams. The two sub-beams are then recombined to produce aninterference pattern which is shone on the core of the optical fiber fora period of time. After the laser beam is turned off, the index ofrefraction variations stay inscribed in the optical fiber.

PCT Application 00/02068 filed on Jun. 30, 1999 by Bhatia et al.describes an apparatus to write Bragg gratings in an optical fiber. Theapparatus includes a laser, which produces a laser beam. The laser beamis split into two sub-beams by a beam splitter. Then, the two sub-beamsare each reflected by a plurality of mirrors to make them converge at acertain location in space. The two converging beams interfere andtherefore produce an interference pattern at the certain location. Theoptical fiber is positioned at the certain location to write thegrating.

Apparatuses such as the one described in the above-referenced PCTapplication present many disadvantages. First, the mirrors have to beprecisely aligned to produce the desired interference pattern. Also, thewhole apparatus has to be very rigid and isolated from externalvibrations to keep the interference pattern at a precise location inspace. If the interference pattern is displaced during the writingprocess, the grating will be veiled and may eventually be useless. Inaddition, the surface of the mirrors has to be kept clean in order tobring as much energy as possible to the certain location where theinterference pattern is produced.

Two properties that are often required of Bragg gratings are apodizationand balance. Apodization relates to having an interference patternincluding a plurality of bright fringes and a plurality of dark fringeswherein the bright fringes are not uniformly bright across the wholeinterference pattern. Therefore, if an apodized interference patternwith fringes having a low intensity close to the extremities of thegrating is used to produce the Bragg grating, the index of refractiondifferences will also be apodized, which is desirable in some Bragggratings used as optical filters. Balance relates to having indices ofrefraction in the grating which vary alternatively above and below anaverage value. In the apparatus described above, only variations inindices of refraction in one direction are possible as dark fringes inthe interference pattern produce no variation in the index of refractioninside the optical fiber and bright fringes all produce variations inthe index of refraction inside the optical fiber having a same sign.Once again, it is often desirable to have variations above and below anaverage value when the gratings are used as optical filters.

To produce a balanced grating, two exposures are required in theapparatus described above. In a first exposure, the beams, eventuallyapodized, are used to create the variations in index of refraction asdescribed above. In the second exposure, the sub-beams are slightlyoffset to provide a uniform increase in index of refraction along thegrating. However, this two-step process is time consuming as twoexposures have to be made. In addition, the apodization is performedthrough collimators and spatial filters which need to be preciselyaligned with the rest of the apparatus.

In view of the above, there is a need in the industry to provide newapparatuses and method for writing features in or on a photosensitivemedium.

More particularly, the invention relates to the use of an interferencepattern between two coherent light beams to induce changes in the indexof refraction of the medium wherein the two light beams are produced bysplitting a first light beam and propagated in a prism through totalinternal reflection.

SUMMARY OF THE INVENTION

According to a first broad aspect, the present invention provides anapparatus for producing an interference pattern from an input opticalbeam. The apparatus includes a first optical element for separating theinput optical beam into a plurality of divergent optical sub-beams and asecond optical element including a first surface and a second surface.The first surface is optically coupled to the first optical element toreceive at least two of the plurality of sub-beams. In addition, thesecond optical element is capable of redirecting via total internalreflection at least one of the sub-beams received at the first surfacesuch that at least two sub-beams emerge from the second surface alongrespective paths intersecting one another outside the second opticalelement at a distance from the second surface.

Advantageously, the use of total internal reflection in the secondoptical element for sub-beam redirection removes any requirement for amirror to perform this function. This improves not only physicalrobustness but also sensitivity to dust and grease. Moreover, there areonly minimal power losses as the sub-beams reflect internally and thelocation at which they intersect is easily adjustable.

In addition, the present invention has the capability to pass zeroth andfirst order diffraction sub-beams, which allows a balanced grating to beproduced in a single exposure.

According to a second broad aspect, the present invention provides amethod of writing an interference pattern on a photosensitive mediumwith a laser beam. The method includes receiving at least two of thesub-beams at an optical element; redirecting at least one of thereceived sub-beams via total internal reflection such that at least twosub-beams emerge from the optical element along respective paths thatintersect in a region of intersection; and placing the photosensitivemedium at least partly in the region of intersection.

These and other aspects and features of the present invention will nowbecome apparent to those of ordinary skill in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows an optical apparatus for producing an interference patternon a photosensitive substrate in accordance with an embodiment of thepresent invention;

FIG. 2 shows a region of space wherein the interference pattern isproduced by the optical apparatus shown in FIG. 1; and

FIG. 3 shows a valiant of the optical apparatus shown on FIG. 1including a curve surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an optical apparatus 100 for producing an interferencepattern on a photosensitive medium in the form of a core 105 of anoptical fiber 110. While the optical apparatus 100 is described hereinin the context of Bragg grating writing in an optical fiber 110, thereader skilled in the art will readily appreciate that the apparatuscould be used in other contexts without departing from the spirit of theinvention. In specific examples of implementation, the apparatus couldbe used to produce an interference pattern illuminating otherphotosensitive media, including discrete optical fibers, optical fibersmounted in a module and integrated optics components.

The optical apparatus 100 includes a laser 120, a diffractive element130 and a transmissive block 140 as described in further detail hereinbelow. The laser 120 produces a coherent beam of light 125. In the caseof Bragg grating writing in an optical fiber, the laser 120 may producelight at a wavelength between 193 nm and 300 nm and is either pulsed orcontinuous. In an even more specific case, the laser 120 produces lightat a wavelength between 193 nin and 260 nm. However, it will beunderstood that a laser 120 producing a beam 125 having a wavelengthoutside of the mentioned interval can be used in the apparatus 100.

The beam 125 may be shaped and collimated by a lens assembly 127. Theshaped and collimated beam is then defected by a mirror 129, that can beused to optimally align the beam 125 with the core 105. The mirror 129may be movable in order to permit precise alignment to be controlled bya user or a feedback control circuit. The reader skilled in the art willrecognize that while the laser 120, the lens assembly 127 and the mirror129 are preferably used in the optical apparatus 100, other sources ofcoherent light could be used without departing from the spirit of theinvention, with or without the lens assembly 127. Also, the apparatus ofthe present invention may be used in cases where it is desirable toproduce an interference pattern from non-coherent light.

The beam 125 arrives at the diffractive element 130, which produces adiffraction pattern including a plurality of sub-beam 135 _(k), k=0, ±1,±2, . . . Each pair of sub-beams 135 _(±k) corresponds to a diffractionorder k of a diffraction pattern produced by the diffractive element130. The sub-beams 135 _(k) diverge from each other, each at arespective divergence angle measured with respect to the sub-beam 135 ₀.In a specific example of implementation, the diffractive element 130 canbe an apodized holographic phase mask producing sub-beams 135 ⁻¹ and135₁ diverging from the sub-beam 135 ₀ at an angle between 7° and 23°.The exact number of sub-beams 135 _(k) produced and the value of theirrespective divergence angle depend on the specific diffractive elementused and on the wavelength of the beam 125 produced by the laser 120.

In a variant, the diffractive element 130 can be replaced by a beamsplitter in the optical apparatus 100. However, a typical beam splitterdoes not produce an order 0 sub-beam 135 ₀. As it will be detailedbelow, the order 0 sub-beam 135 ₀ is preferably present to produce abalanced grating in a single exposure.

The transmissive block 140 is composed of a material having an index ofrefraction higher than its surroundings and which is transparent ornearly transparent at the wavelenth of the beam 125 produced by thelaser 120. In a very specific example of implementation, thetransmissive block 140 can be made of quartz. In a specific example ofimplementation, shown on FIG. 1, the transmissive block 140 is a cubicprism having homogenous optical properties and including two planarlateral faces 141 and 142, one planar front face 143 and one planar backforce 144. In a very specific example of implementation, suitabledimensions of the cubic prism may be 3 cm×3 cm×15 cm locatedapproximately 2 cm from the diffractive element 130 and approximately 2cm from the core 105. However, the reader skilled in the art willappreciate that these dimensions can vary considerably, depending on theinterference pattern to be produced.

In an embodiment of the present invention, the transmissive block 140 isadapted to propagate only the zeroth and first orders of diffractionproduced by the diffractive element 130, namely, the sub-beams 135 ⁻¹,135 ₀ and 135 ₁. Sub-beams corresponding to other orders of diffractioncan be avoided by suitably dimensioning and positioning the transmissiveblock 140 so that it is clear of the path taken by the sub-beamscorresponding to these other orders of diffraction. In other embodimentsof the invention, undesired orders of diffraction are filtered by thetransmission block 140.

In one embodiment of the present invention, the two first ordersub-beams 135 _(±1) are reflected inside the transmissive block 140through total internal reflection and subsequently converge on the core105 to produce an interference pattern. In other embodiments of thepresent invention, one of the first order sub-beams may pass straightthrough the transmissive block 140, while the other of the first ordersub-beams may be totally internally reflected and redirected towards thesub-beam that was not totally internally reflected. Intersection of atleast two sub-beams exiting a back surface of the transmissive block 140occurs outside the transmissive block 140 at a distance away from itsback surface.

In this specific example of implementation, the sub-beams 135 ⁻¹, 135 ₀and 135 ₁ enter the transmissive block 140 through the front face 143.Since the sub-beams 135 ⁻¹ and 135 ₁ are not perpendicular to the frontface 143, they will be refracted when entering the transmissive block140, in opposition to the sub-beam 135 ₀ which enters the transmissiveblock 140 perpendicularly to the front face 143 and is therefore notrefracted.

Inside the transmissive block 140, the sub-beam 135 ₀ is propagated in astraight line to the back face 144. However, the dimensions of thetransmissive block 140 are such that the two sub-beams 135 ⁻¹ and 135 ₁arrive to the lateral faces 141 and 142 before arriving to the back face144. Since the index of refraction inside the transmissive block 140 islarger than the index of refraction outside the transmissive block 140,the two sub-beams 135 ⁻¹ and 135 ₁ are reflected through total internalreflection at the lateral surfaces 141 and 142 of the transmissive block140. Also, the transmissive block 140 has dimensions such that the twosub-beams 135 ⁻¹ and 135 ₁ will arrive to the back face 144 beforeintersecting.

When exiting the transmissive block 140 through the back face 144, thetwo sub-beams 135 ⁻¹ and 135 ₁ are refracted and converge at a certainlocation in space. Since the two sub-beams 135 ⁻¹ and 135 ₁ have beenreflected inside a single rigid piece of material, there are onlyminimal losses in a power carried by the two sub-beams 135 ⁻¹ and 135 ₁and the location at which they intersect is easily adjustable.

Meanwhile, the zeroth order sub-beam 135 ₀ emerges from the transmissiveblock 140 without having been deflected and the zeroth order sub-beammay be focused by a focusing lens 150. The relative position of thefocusing lens 150 with respect to the optical fiber 110 determines anintensity of the zeroth order beam 135 ₀ illuminating the optical fiber110, which allows to write a balanced grating on the optical fiber in asingle exposure. The reader skilled in the art will readily appreciatethat the focussing lens 150 alleviates the need for a specializeddiffractive element that is capable of producing balanced order 1 and 0sub-beams 135 ⁻¹, 135 ₀ and 135 ₁.

The reader skilled in the art will readily appreciate that many shapesof the transmissive block 140 can be designed so as to select onlyorders 0 and 1 of diffraction and make two sub-beams of first orderconverge at the certain location in space through total internalreflection. In addition, transmissive blocks selecting other orders ofdiffraction can be used in the optical apparatus 100 without departingfrom the spirit of the invention.

In a variant, the front face 143 of the transmissive block 140 ispartially coated with an opaque layer to block the sub-beam 135 ₀. Thismay be desirable in processes wherein the sub-beam 135 ₀ is notrequired.

It will be appreciated that since the transmissive block 140 is aself-contained unit for redirecting the sub-beams 135 _(k), it can bereadily exchanged with another transmissive block with only minimalrealignment requirements, which affords flexibility in the use of theapparatus 100.

It will also be appreciated that the distance between the focusing lens150 and the core 105 determines the intensity of the sub-beam 135 ₀ atthe location of the core 105. Alternatively, the focusing lens 150 couldbe interchanged with another lens having a different focal distance tovary intensity of the sub-beam 135 ₀ at the location of the core 105.The reader skilled in the art will appreciate that the exact value ofthe distance between the focusing lens 150 and the core 105 and theexact value of the focal distance of the focusing lens 150 required toproduce a balanced Bragg grating depend on many characteristics of theapparatus 100. Accordingly, it is preferable to adjust these parametersfor each particular grating written, either through theoreticalcalculations or through measurements of intensity using an optical powermeter. Such methods for adjusting these parameters are well known in theart and will not be discussed in further detail.

It will further be appreciated that in those instances when the sub-beam135 ₀ is undesired, the sub-beam 135 ₀ can be blocked by replacing thefocussing lens 150 by a piece of all opaque material.

It will also be appreciated that the distance between the transmissiveblock 140 and the core 105 regulates a length of grating written in theoptical fiber 110. As shown on FIG. 2, the interference pattern producedby the two sub-beams 135 ⁻¹ and 135 ₁ is present in a diamond-shapedregion of space 210 in which the two sub-beams 135 ⁻¹ and 135 ₁intersect. Depending on the exact position of the core 105 in thediamond-shaped region of space 210, the length of a portion of the core105 exposed to the interference pattern will vary, which will thereforechange the length of the grating produced.

In a further variant, shown on FIG. 3, the transmissive block 140includes a curved surface 197A instead of the front face 143 shown inFIG. 1, which was planar. Backward or forward shifting the curvedsurface 197A can be used to change the angle at which the sub-beams 135⁻¹ and 135 ₁ enter the transmissive block 140, which changes the angleat which the sub-beams 135 ⁻¹ and 135 ₁ leave the transmissive block140, which changes the period of the Bragg grating produced at thelocation of intersection of the sub-beams 135 ⁻¹ and 135 ₁.

Specifically, changing the distance between the curved surface 197 andthe diffractive element 130 (i.e., moving from A to B in FIG. 3)produces a variation in the location along the surface 197 at which thedivergent sub-beams 135 ⁻¹ and 135 ₁ enter the transmissive block 140.Due to the surface 197 not being planar, the angle of incidence withwhich the sub-beams 135 ⁻¹ and 135 ₁ arrive at the curved surface 197varies with the distance between the curved surface 197 and thediffractive element 130. This variation in the angle of incidence atwhich the sub-beams 135 ⁻¹ and 135 ₁ enter the transmissive block 140produces a variation in the angle at which the two sub-beams 135 ⁻¹ and135 ₁ are propagated in the transmissive block 140 further to beingrefracted at the surface 197.

This may lead to an intersection occurring at a different region inspace for case A than case B. However, the distance between the backface 144 and the region of intersection can be controlled byappropriately selecting the distance between the diffractive element 130and the curved surface 197. Specifically, by appropriately shifting theposition of the diffractive element, the sub-beams 135 ⁻¹ and 135 ₁exiting the transmissive block 140 can be made to intersect at the sameregion in space in both A and B. In this way, it is possible to changethe period of the Bragg grating by merely changing the transmissiveblock 140 without having to change any other component in the apparatus100.

It should be appreciated that in some embodiments, the transmissiveblock 140 may be composed of a basic block in the shape of a prism, towhich it is possible to optically couple any of a set of curvedattachment blocks, each having a curved face and a particular length.The curved face may have the same curvature profile or it may bedifferent for different attachment blocks of different lengths. Also, itis within the scope of the present invention to provide attachmentblocks of roughly the same length, with different curvature profiles inorder to achieve different angles of intersection and hence differentBragg periodicity. Those skilled in the art will be capable ofdetermining what shift, if any, is required in the position of thediffraction element 130 in order to maintain the distance between thetransmissive block 140 and the region of intersection of the first ordersub-beams.

Those skilled in the art will appreciate that the apparatus 100 wouldwork in sensibly the same way if the inside walls of the transmissiveblock 140 are not parallel or if the curved surface 197 is located onthe side of the transmissive block 140 through which the light exits thetransmissive block 140.

While specific embodiments of the present invention have been describedand illustrated, it will be apparent to those skilled in the art thatnumerous modifications and variations can be made without departing fromthe scope of the invention as defined in the appended claims.

1-51. (canceled)
 52. Apparatus for writing a grating on a waveguide,comprising: a) a block of light transmissive material, said blockincluding: i) a surface for receiving a plurality of optical beams forpropagation through said block; ii) a reflective element for reflectingoptical energy in a first one of the optical beams to produce areflected beam such that the reflected beam is directed to exit saidblock and continue propagating toward the waveguide and intersect asecond one of the optical beams outside of said block.
 53. Apparatus asdefined in claim 52, wherein said surface is a first surface, saidreflective element includes a second surface of said block of lighttransmissive material.
 54. Apparatus as defined in claim 53, wherein thewaveguide is an optical fiber.
 55. Apparatus as defined in claim 54,wherein each of the optical beams is laser light.
 56. Apparatus asdefined in claim 53, wherein said reflective element is a firstreflective element, said block of light transmissive material includinga second reflective element that is part of said block of lighttransmissive material for reflecting the second one of the opticalbeams.
 57. Apparatus as defined in claim 56, wherein said secondreflective element includes a third surface of said block of lighttransmissive material.
 58. Apparatus as defined in claim 57, whereinsaid first surface and said third surface are positioned forrespectively reflecting the first one of the optical beams and thesecond one of the optical beams such that the first one of the opticalbeams intersects the second one of the optical beams at a certaindistance away from said block of light transmissive material to createan interference pattern on the waveguide.
 59. Apparatus as defined inclaim 52, further comprising an optical element for receiving an inputoptical beam and separating the input optical beam into the plurality ofoptical beams.
 60. Method for writing a grating on a waveguide,comprising: a) directing a plurality of optical beams toward a firstsurface of a block of light transmissive material for propagationthrough said block; and b) while a first one of the optical beams ispropagating through said block of light transmissive material,reflecting optical energy from the first one of the optical beams toproduce a reflected beam that also propagates through said block and isdirected to exit said block and continue propagating toward thewaveguide and intersect a second one of the optical beams outside ofsaid block.
 61. Method as defined in claim 60, wherein the waveguide isan optical fiber.
 62. Method as defined in claim 61, wherein each of theoptical beams is laser light.
 63. Method as defined in claim 60, furthercomprising reflecting the second one of the optical beams while thesecond one of the optical beams is propagating through said block. 64.Method as defined in claim 63, wherein reflecting the first one of theoptical beams and the second one of the optical beams causes the firstone of the optical beams to intersect the second one of the opticalbeams at a certain distance away from the block of light transmissivematerial to create an interference pattern on the waveguide. 65.Apparatus for writing a grating on a waveguide, said apparatuscomprising a block of light transmissive material, said block includingfirst and second reflective elements integrated to said block, saidfirst and second reflective elements being located one with respect tothe other such that when first and second optical beams that propagatethrough said block of light transmissive material impinge upon the firstand second reflective elements, respectively, the first and secondoptical beams are directed to exit said block of light transmissivematerial toward the waveguide and intersect each other outside saidblock of light transmissive material.
 66. Apparatus as defined in claim65, wherein said first and second reflective elements are respectivesurfaces of said block of light transmissive material.
 67. Apparatus asdefined in claim 66, wherein said respective surfaces are opposite eachother.
 68. Apparatus as defined in claim 65, wherein the waveguide is anoptical fiber.
 69. Apparatus as defined in claim 68, wherein the firstand second optical beams form an interference pattern on the opticalfiber.